Six steps to hell

By the end of the century, the Earth could be more than 6C hotter than it is today, according to the Intergovernmental Panel on Climate Change. We know that would be bad news - but just how bad? How big a rise will it take for the Alps to melt, the oceans to die and desert to conquer Europe and the Americas? Mark Lynas sifted through thousands of scientific papers for his new book on global warming. This is what the research told him ...

A dead pelican in a drought in Mexico. Photograph: Eduardo Resendiz/EPA

Nebraska isn't at the top of most tourists' to-do lists. However, this dreary expanse of impossibly flat plains sits in the middle of one of the most productive agricultural systems on Earth. Beef and corn dominate the economy, and the Sand Hills region - where low, grassy hillocks rise up from the flatlands - has some of the best cattle ranching in the whole US. But scratch beneath the grass and you will find, as the name suggests, not soil but sand. These innocuous-looking hills were once desert, part of an immense system of sand dunes that spread across the Great Plains from Texas in the south to the Canadian prairies in the north. Six thousand years ago, when temperatures were about 1C warmer than today in the US, these deserts may have looked much as the Sahara does today. As global warming bites, the western US could once again be plagued by perennial drought - devastating agriculture and driving out human inhabitants on a scale far larger than the 1930s "Dustbowl" exodus.

On the other side of the Atlantic, today's hottest desert could be seeing a wetter future in the one-degree world. At the same time as sand dunes were blowing across the western US, the central Sahara was a veritable Garden of Eden as rock paintings of elephants, giraffes and buffalo, also dating from 6,000 years ago, attest. On the borders of what is today Chad, Nigeria and Cameroon, the prehistoric Lake Mega-Chad spread over an area only slightly smaller than the Caspian Sea does now. Could a resurgent north African monsoon drive rainfall back into the Sahara in a one-degree world? Models suggest it could.

Also in Africa, Mount Kilimanjaro will be losing the last of its snow and ice as temperatures rise, leaving the entire continent ice-free for the first time in at least 11,000 years. The Alps, too, will be melting, releasing deadly giant landslides as thawing permafrost removes the "glue" that holds the peaks together. In the Arctic, temperatures will rise far higher than the one-degree global average, continuing the rapid decline in sea ice that scientists have already observed. This spells bad news for polar bears, walruses and ringed seals - species that are effectively pushed off the top of the planet as warming shrinks cold areas closer and closer to the pole.

Indeed, it is the ecological effects of warming that may be most apparent at one degree. Critically, this temperature rise may wipe out the majority of the world's tropical coral reefs, devastating marine biodiversity. Most of the Great Barrier Reef will be dead.

In the highly unlikely event that global warming deniers prove to be right, we will still have to worry about carbon dioxide, because it dissolves in the oceans and makes them more acidic. Even with relatively low emissions, large areas of the southern oceans and parts of the Pacific will within a few decades become toxic to organisms with calcium carbonate shells, for the simple reason that the acidic seawater will dissolve them. Many species of plankton - the basis of the marine food chain and essential for the sustenance of higher creatures, from mackerel to baleen whales - will be wiped out, and the more acidic seawater may be the knockout blow for what remains of the world's coral reefs. The oceans may become the new deserts as the world's temperatures reach 2C above today's.

Two degrees may not sound like much, but it is enough to make every European summer as hot as 2003, when 30,000 people died from heatstroke. That means extreme summers will be much hotter still. As Middle East-style temperatures sweep across Europe, the death toll may reach into the hundreds of thousands. The Mediterranean area can expect six more weeks of heatwave conditions, with wildfire risk also growing. Water worries will be aggravated as the southern Med loses a fifth of its rainfall, and the tourism industry could collapse as people move north outside the zones of extreme heat.

Two degrees is also enough to cause the eventual complete melting of the Greenland ice sheet, which would raise global sea levels by seven metres. Much of the ice-cap disappeared 125,000 years ago, when global temperatures were 1-2C higher than now. Because of the sheer size of the ice sheet, no one expects this full seven metres to come before the end of the century, but a top Nasa climate scientist, James Hansen, is warning that the mainstream projections of sea level rise (of 50cm or so by 2100) could be dangerously conservative. As if to underline Hansen's warning, the rate of ice loss from Greenland has tripled since 2004.

This melting will also continue to affect the world's mountain ranges, and in Peru all the glaciers will disappear from the Andean peaks that currently supply Lima with water. In California, the loss of snowpack from the Sierra Nevada - three-quarters of which could disappear in the two-degree world - will leave cities such as Los Angeles increasingly thirsty during the summer. Global food supplies, especially in the tropics, will also be affected but while two degrees of warming will be survivable for most humans, a third of all species alive today may be driven to extinction as climate change wipes out their habitat.

Scientists estimate that we have at best 10 years to bring down global carbon emissions if we are to stabilise world temperatures within two degrees of their present levels. The impacts of two degrees warming are bad enough, but far worse is in store if emissions continue to rise. Most importantly, 3C may be the "tipping point" where global warming could run out of control, leaving us powerless to intervene as planetary temperatures soar. The centre of this predicted disaster is the Amazon, where the tropical rainforest, which today extends over millions of square kilometres, would burn down in a firestorm of epic proportions. Computer model projections show worsening droughts making Amazonian trees, which have no evolved resistance to fire, much more susceptible to burning. Once this drying trend passes a critical threshold, any spark could light the firestorm which destroys almost the entire rainforest ecosystem. Once the trees have gone, desert will appear and the carbon released by the forests' burning will be joined by still more from the world's soils. This could boost global temperatures by a further 1.5ºC - tippping us straight into the four-degree world.

Three degrees alone would see increasing areas of the planet being rendered essentially uninhabitable by drought and heat. In southern Africa, a huge expanse centred on Botswana could see a remobilisation of old sand dunes, much as is projected to happen earlier in the US west. This would wipe out agriculture and drive tens of millions of climate refugees out of the area. The same situation could also occur in Australia, where most of the continent will now fall outside the belts of regular rainfall.

With extreme weather continuing to bite - hurricanes may increase in power by half a category above today's top-level Category Five - world food supplies will be critically endangered. This could mean hundreds of millions - or even billions - of refugees moving out from areas of famine and drought in the sub-tropics towards the mid-latitudes. In Pakistan, for example, food supplies will crash as the waters of the Indus decline to a trickle because of the melting of the Karakoram glaciers that form the river's source. Conflicts may erupt with neighbouring India over water use from dams on Indus tributaries that cross the border.

In northern Europe and the UK, summer drought will alternate with extreme winter flooding as torrential rainstorms sweep in from the Atlantic - perhaps bringing storm surge flooding to vulnerable low-lying coastlines as sea levels continue to rise. Those areas still able to grow crops and feed themselves, however, may become some of the most valuable real estate on the planet, besieged by millions of climate refugees from the south.

At four degrees another tipping point is almost certain to be crossed; indeed, it could happen much earlier. (This reinforces the determination of many environmental groups, and indeed the entire EU, to bring us in within the two degrees target.) This moment comes as the hundreds of billions of tonnes of carbon locked up in Arctic permafrost - particularly in Siberia - enter the melt zone, releasing globally warming methane and carbon dioxide in immense quantities. No one knows how rapidly this might happen, or what its effect might be on global temperatures, but this scientific uncertainty is surely cause for concern and not complacency. The whole Arctic Ocean ice cap will also disappear, leaving the North Pole as open water for the first time in at least three million years. Extinction for polar bears and other ice-dependent species will now be a certainty.

The south polar ice cap may also be badly affected - the West Antarctic ice sheet could lift loose from its bedrock and collapse as warming ocean waters nibble away at its base, much of which is anchored below current sea levels. This would eventually add another 5m to global sea levels - again, the timescale is uncertain, but as sea level rise accelerates coastlines will be in a constant state of flux. Whole areas, and indeed whole island nations, will be submerged.

In Europe, new deserts will be spreading in Italy, Spain, Greece and Turkey: the Sahara will have effectively leapt the Straits of Gibraltar. In Switzerland, summer temperatures may hit 48C, more reminiscent of Baghdad than Basel. The Alps will be so denuded of snow and ice that they resemble the rocky moonscapes of today's High Atlas - glaciers will only persist on the highest peaks such as Mont Blanc. The sort of climate experienced today in Marrakech will be experienced in southern England, with summer temperatures in the home counties reaching a searing 45C. Europe's population may be forced into a "great trek" north.

To find out what the planet would look like with five degrees of warming, one must largely abandon the models and venture far back into geological time, to the beginning of a period known as the Eocene. Fossils of sub-tropical species such as crocodiles and turtles have all been found in the Canadian high Arctic dating from the early Eocene, 55 million years ago, when the Earth experienced a sudden and dramatic global warming. These fossils even show that breadfruit trees were growing on the coast of Greenland, while the Arctic Ocean saw water temperatures of 20C within 200km of the North Pole itself. There was no ice at either pole; forests were probably growing in central Antarctica.

The Eocene greenhouse event fascinates scientists not just because of its effects, which also saw a major mass extinction in the seas, but also because of its likely cause: methane hydrates. This unlikely substance, a sort of ice-like combination of methane and water that is only stable at low temperatures and high pressure, may have burst into the atmosphere from the seabed in an immense "ocean burp", sparking a surge in global temperatures (methane is even more powerful as a greenhouse gas than carbon dioxide). Today vast amounts of these same methane hydrates still sit on subsea continental shelves. As the oceans warm, they could be released once more in a terrifying echo of that methane belch of 55 million years ago. In the process, moreover, the seafloor could slump as the gas is released, sparking massive tsunamis that would further devastate the coasts.

Again, no one knows how likely this apocalyptic scenario is to unfold in today's world. The good news is that it could take centuries for warmer water to penetrate down to the bottom of the oceans and release the stored methane. The bad news is that it could happen much sooner in shallower seas that see a stronger heating effect (and contain lots of methane hydrate) such as in the Arctic. It is also important to realise that the early Eocene greenhouse took at least 10,000 years to come about. Today we could accomplish the same feat in less than a century.

If there is one episode in the Earth's history that we should try above all not to repeat, it is surely the catastrophe that befell the planet at the end of the Permian period, 251 million years ago. By the end of this calamity, up to 95% of species were extinct. The end-Permian wipeout is the nearest this planet has ever come to becoming just another lifeless rock drifting through space. The precise cause remains unclear, but what is undeniable is that the end-Permian mass extinction was associated with a super-greenhouse event. Oxygen isotopes in rocks dating from the time suggest that temperatures rose by six degrees, perhaps because of an even bigger methane belch than happened 200 million years later in the Eocene.

Sedimentary layers show that most of the world's plant cover was removed in a catastrophic bout of soil erosion. Rocks also show a "fungal spike" as plants and animals rotted in situ. Still more corpses were washed into the oceans, helping to turn them stagnant and anoxic. Deserts invaded central Europe, and may even have reached close to the Arctic Circle.

One scientific paper investigating "kill mechanisms" during the end-Permian suggests that methane hydrate explosions "could destroy terrestrial life almost entirely". Acting much like today's fuel-air explosives (or "vacuum bombs"), major oceanic methane eruptions could release energy equivalent to 10,000 times the world's stockpile of nuclear weapons.

Whatever happened back then to wipe out 95% of life on Earth must have been pretty serious. And while it would be wrong to imagine that history will ever straightforwardly repeat itself, we should certainly try and learn the lessons of the distant past. If they tell us one thing above all, it is this: that we mess with the climatic thermostat of this planet at our extreme - and growing - peril.

· Mark Lynas's Six Degrees: Our Future on a Hotter Planet is published by Fourth Estate, priced £12.99. To order a copy for £11.99 with free UK p&p go to Guardian.co.uk/bookshop or call 0870 836 0875